Implantable cardiac stimulation device and method providing dynamic sensing configurations for bichamber stimulation and tachyarrhythmia detection

ABSTRACT

An implantable cardiac stimulation device provides bichamber pacing and dynamic bichamber and single chamber sensing. The device includes a sensing circuit that senses activity of a heart, a lead system coupled to a plurality of chambers of the heart, and a cardiac rate circuit that determines a cardiac rate of the heart. A control circuit causes the lead system to couple the sensing circuit to corresponding chambers of the heart to enable bichamber trigger pacing when the cardiac rate is below a given rate and to a single chamber of the heart when the cardiac rate is above the given rate to enable enhanced tachycardia sensing.

FIELD OF THE INVENTION

The present invention generally relates to an implantable cardiacstimulation device that provides electrical therapy to a patient'sheart. The present invention more particularly relates to such a devicethat provides bichamber pacing and tachyarrhythmia detection withselectable sensing electrode configurations.

BACKGROUND OF THE INVENTION

Implantable cardiac devices are well known in the art. They may take theform of implantable defibrillators or cardioverters which treataccelerated rhythms of the heart such as fibrillation or implantablepacemakers which maintain the heart rate above a prescribed limit, suchas, for example, to treat a bradycardia. Implantable cardiac devices arealso known which incorporate both a pacemaker and a defibrillator.

A pacemaker may be considered to be comprised of two major components.One component is a pulse generator which generates the pacingstimulation pulses and includes the electronic circuitry and the powercell or battery. The other component is the lead, or leads, havingelectrodes which electrically couple the pacemaker to the heart. A leadmay provide both unipolar and bipolar pacing and/or sensing electrodeconfigurations. In the unipolar configuration, the pacing stimulationpulses are applied or intrinsic responses are sensed between a singleelectrode carried by the lead, in electrical contact with the desiredheart chamber, and the pulse generator case. The electrode serves as thecathode (negative pole) and the case serves as the anode (positivepole). In the bipolar configuration, the pacing stimulation pulses areapplied or intrinsic responses are sensed between a pair of closelyspaced electrodes carried by the lead, in electrical contact with thedesired heart chamber, with the most proximal electrode serving as theanode and the most distal electrode serving as the cathode.

Pacemakers deliver pacing pulses to the heart to induce a depolarizationand a mechanical contraction of that chamber when the patient's ownintrinsic rhythm fails. To this end, pacemakers include sensing circuitsthat sense cardiac activity for the detection of intrinsic cardiacevents such as intrinsic atrial events (P waves) and intrinsicventricular events (R waves). By monitoring such P waves and/or R waves,the pacemaker circuits are able to determine the intrinsic rhythm of theheart and provide stimulation pacing pulses that force atrial and/orventricular depolarizations at appropriate times in the cardiac cyclewhen required to help stabilize the electrical rhythm of the heart.

Pacemakers are described as single-chamber or dual-chamber systems. Asingle-chamber system stimulates and senses in one chamber of the heart(atrium or ventricle). A dual-chamber system stimulates and/or senses inboth chambers of the heart (atrium and ventricle). Dual-chamber systemsmay typically be programmed to operate in either a dual-chamber mode ora single-chamber mode.

Recently, there has been the introduction of pacing systems thatstimulate in corresponding chambers of the heart as, for example, theright ventricle (RV) and left ventricle (LV). These are termedbiventricular stimulation devices.

Biventricular pacing has been shown to coordinate contractions of theleft and right ventricles, reduce the amount of blood flow that leaksthrough the mitral valve, and decreases the motion of the septal wallthat separates the chambers of the heart. Such motion can affect thequantity of blood that the ventricle can pump out in a single beat.

Biventricular pacing has been found to be particularly advantageous inpatients suffering from congestive heart disease because of the improvedability of the left ventricle to fully pump blood from the heart. As aresult, patients are able to tolerate greater exertion, have a longerlife span, and experience a higher quality of life.

Biatrial pacing has also been suggested to also lend in coordinatingcontractions of the right and left atria. As used herein, the termcorresponding chambers is meant to refer to either the combination ofthe right and left atria or the combination of the right and leftventricles.

One form of biventricular pacing is referred to as CardiacResynchronization Therapy (CRT). It has been shown to have a particularpositive effect on patients with heart failure (HF). There are a numberof ways CRT may be performed. For example, during an atrial trackingmode (DDD), the AV/PV intervals may be set to be short, a negativehysteresis value can be used to ensure pacing, or a trigger mode may beestablished to provide a pacing stimulation pulse upon a sensed event.For non-tracking modes (DDI, VVI) a high ventricular pacing rate ortriggered pacing may be used.

Triggered pacing offers the patient a more physiologic AV/PV delay (asit is their own intrinsic rate) and ensures high percentage of pacingduring times of atrial tachycardia (AT) or atrial fibrillation (AF), acritical time for HF patients to receive CRT therapy. However, fortriggered pacing to be truly effective in synchronization, the sensingshould come from the left and right ventricles. Additionally, sensingfrom both of these corresponding chambers will ensure that longconduction delays or premature ventricular contractions (PVCs) do notcause extra harm from pacing into a vulnerable period that may come fromsensing from one chamber only. For example, a PVC may occur on the leftside and be sensed over 100 ms later in the right side to which thedevice would elicit a biventricular pace pulse into a potentialvulnerable period.

Using a combined sensing configuration (left and right ventricle)however poses a problem with accurate ventricular tachycardia andfibrillation detection. For example, if the patient has a very wide QRScomplex (which is very likely with an HF patient), then combined rightand left sensing may cause the device to double count, thus delivering amore aggressive therapy than may be necessary, i.e., fibrillationtherapy instead of antitachycardia pacing (ATP) therapy, or even aninappropriate therapy.

For devices with independent right and left sensing, this may not posesuch a problem. But for devices that have limited sense channels, anapproach that could encompass right and left sensing while pacing andright sensing while in a tachycardia would be preferred. The presentinvention addresses this and other issues.

SUMMARY OF THE INVENTION

The invention provides an implantable cardiac stimulation devicecomprising a sensing circuit that senses activity of a heart, a leadsystem coupled to a plurality of chambers of the heart, and a cardiacrate circuit that determines a cardiac rate of the heart. The devicefurther comprises a control circuit that causes the lead system tocouple the sensing circuit to corresponding chambers of the heart whenthe cardiac rate is below a given rate and to a single chamber of theheart when the cardiac rate is above the given rate.

The device may further comprise a pulse generator that provides pacingpulses to the corresponding chambers of the heart. The pulse generatormay provide simultaneous pacing pulses or sequential pacing pulses tothe corresponding chambers of the heart. The pulse generator may providethe pacing pulses in a trigger mode up to a given rate limit which maybe a maximum trigger rate.

The corresponding chambers may be the right ventricle and the leftventricle of the heart. The single chamber may be one of the ventriclesof the heart. More particularly, the single chamber may be the rightventricle.

The corresponding chambers may alternatively be the right atrium and theleft atrium of the heart. The single chamber may then be one of theatria of the heart. More particularly, the single chamber may be theright atrium.

The cardiac rate circuit may determine the cardiac rate for eachheartbeat and the control circuit may cause the lead system totransition from coupling the sensing circuit to the single chamber tocoupling the sensing circuit to the corresponding chambers of the heartafter the rate has been below the given rate for a predetermined numberof heartbeats.

The invention further provides an implantable cardiac stimulation devicecomprising a sensing circuit that senses cardiac activity of a heart, alead system coupled to a plurality of chambers of the heart, and a pulsegenerator that provides pacing pulses to corresponding chambers of theheart responsive to the sensing circuit. The device further comprises acardiac rate circuit that determines a cardiac rate of the heart, and acontrol circuit that causes the lead system to couple the sensingcircuit to the corresponding chambers of the heart when the rate isbelow a maximum rate and to only one of the corresponding chambers whenthe rate is above the maximum rate.

The invention still further provides a method for use in an implantablecardiac stimulation device. The method comprises determining a cardiacrate associated with interaction of the device and the heart and sensingcardiac activity of the heart. The sensing step includes a first sensingstep of sensing cardiac activity of corresponding chambers of the heartwhen the cardiac rate is below a given rate and a second sensing step ofsensing cardiac activity of a single chamber of the heart when thecardiac rate is above the given rate.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention may be morereadily understood by reference to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a simplified diagram illustrating an implantable stimulationdevice according to an embodiment of the invention in electricalcommunication with a patient's heart for delivering multi-chamberstimulation and shock therapy;

FIG. 2 is a functional block diagram of the implantable stimulationdevice of FIG. 1; and

FIG. 3 is a flow chart describing an overview of the operation of oneembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best mode presently contemplated forpracticing the invention. This description is not to be taken in alimiting sense but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe ascertained with reference to the issued claims. In the descriptionof the invention that follows, like numerals or reference designatorswill be used to refer to like parts or elements throughout.

As shown in FIG. 1, there is a stimulation device 10 in electricalcommunication with a patient's heart 12 by way of three leads, 20, 24and 30, suitable for delivering multi-chamber stimulation and shocktherapy. To sense atrial cardiac signals and to provide right atrialchamber stimulation therapy, the stimulation device 10 is coupled to animplantable right atrial lead 20 having at least an atrial tip electrode22, which typically is implanted in the patient's right atrialappendage.

To sense left atrial and ventricular cardiac signals and to provide leftchamber pacing therapy, the stimulation device 10 is coupled to a“coronary sinus” lead 24 designed for placement in the “coronary sinusregion” via the coronary sinus ostium for positioning a distal electrodeadjacent to the left ventricle and/or additional electrode(s) adjacentto the left atrium. As used herein, the phrase “coronary sinus region”refers to the vasculature of the left ventricle, including any portionof the coronary sinus, great cardiac vein, left marginal vein, leftposterior ventricular vein, middle cardiac vein, and/or small cardiacvein or any other cardiac vein accessible by the coronary sinus.Accordingly, an exemplary coronary sinus lead 24 is designed to receiveatrial and ventricular cardiac signals and to deliver left ventricularpacing therapy using at least a left ventricular tip electrode 26, leftatrial pacing therapy using at least a left atrial ring electrode 27,and shocking therapy using at least a left atrial coil electrode 28.

The stimulation device 10 is also shown in electrical communication withthe patient's heart 12 by way of an implantable right ventricular lead30 having, in this embodiment, a right ventricular tip electrode 32, aright ventricular ring electrode 34, a right ventricular (RV) coilelectrode 36, and an SVC coil electrode 38. Typically, the rightventricular lead 30 is transvenously inserted into the heart 12 so as toplace the right ventricular tip electrode 32 in the right ventricularapex so that the RV coil electrode will be positioned in the rightventricle and the SVC coil electrode 38 will be positioned in thesuperior vena cava. Accordingly, the right ventricular lead 30 iscapable of receiving cardiac signals, and delivering stimulation in theform of pacing and shock therapy to the right ventricle.

As illustrated in FIG. 2, a simplified block diagram is shown of themulti-chamber implantable stimulation device 10, which is capable oftreating both fast and slow arrhythmias with stimulation therapy,including cardioversion, defibrillation, and pacing stimulation. While aparticular multi-chamber device is shown, this is for illustrationpurposes only, and one of skill in the art could readily duplicate,eliminate or disable the appropriate circuitry in any desiredcombination to provide a device capable of treating the appropriatechamber(s) with cardioversion, defibrillation and pacing stimulation.

The housing 40 for the stimulation device 10, shown schematically inFIG. 2, is often referred to as the “can”, “case” or “case electrode”and may be programmably selected to act as the return electrode for all“unipolar” modes. The housing 40 may further be used as a returnelectrode alone or in combination with one or more of the coilelectrodes, 28, 36 and 38, for shocking purposes. The housing 40 furtherincludes a connector (not shown) having a plurality of terminals, 42,44, 46, 48, 52, 54, 56, and 58 (shown schematically and, forconvenience, the names of the electrodes to which they are connected areshown next to the terminals). As such, to achieve right atrial sensingand pacing, the connector includes at least a right atrial tip terminal(A_(R) TIP) 42 adapted for connection to the atrial tip electrode 22.

To achieve left chamber sensing, pacing and shocking, the connectorincludes at least a left ventricular tip terminal (V_(L) TIP) 44, a leftatrial ring terminal (A_(L) RING) 46, and a left atrial shockingterminal (A_(L) COIL) 48, which are adapted for connection to the leftventricular ring electrode 26, the left atrial tip electrode 27, and theleft atrial coil electrode 28, respectively.

To support right chamber sensing, pacing and shocking, the connectorfurther includes a right ventricular tip terminal (V_(R) TIP) 52, aright ventricular ring terminal (V_(R) RING) 54, a right ventricularshocking terminal (R_(V) COIL) 56, and an SVC shocking terminal (SVCCOIL) 58, which are adapted for connection to the right ventricular tipelectrode 32, right ventricular ring electrode 34, the RV coil electrode36, and the SVC coil electrode 38, respectively.

At the core of the stimulation device 10 is a programmablemicrocontroller 60 which controls the various modes of stimulationtherapy. As is well known in the art, the microcontroller 60 typicallyincludes a microprocessor, or equivalent control circuitry, designedspecifically for controlling the delivery of stimulation therapy and mayfurther include RAM or ROM memory, logic and timing circuitry, statemachine circuitry, and I/O circuitry. Typically, the microcontroller 60includes the ability to process or monitor input signals (data) ascontrolled by a program code stored in a designated block of memory. Thedetails of the design and operation of the microcontroller 60 are notcritical to the present invention. Rather, any suitable microcontroller60 may be used that carries out the functions described herein. The useof microprocessor-based control circuits for performing timing and dataanalysis functions are well known in the art.

As shown in FIG. 2, an atrial pulse generator 70 and a ventricular pulsegenerator 72 generate pacing stimulation pulses for delivery by theright atrial lead 20, the right ventricular lead 30, and/or the coronarysinus lead 24 via an electrode configuration switch 74. It is understoodthat in order to provide stimulation therapy in each of the fourchambers of the heart, the atrial and ventricular pulse generators, 70and 72, may include dedicated, independent pulse generators, multiplexedpulse generators, or shared pulse generators. The pulse generators, 70and 72, are controlled by the microcontroller 60 via appropriate controlsignals, 76 and 78, respectively, to trigger or inhibit the stimulationpulses.

The microcontroller 60 further includes timing control circuitry 79which is used to control the timing of such stimulation pulses (e.g.,pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A-A)delay, or ventricular interconduction (V-V) delay, etc.) as well as tokeep track of the timing of refractory periods, blanking intervals,noise detection windows, evoked response windows, alert intervals,marker channel timing, etc., which is well known in the art.

The switch 74 includes a plurality of switches for connecting thedesired electrodes to the appropriate I/O circuits, thereby providingcomplete electrode programmability. Accordingly, the switch 74, inresponse to a control signal 80 from the microcontroller 60, determinesthe polarity of the stimulation pulses (e.g., unipolar, bipolar,combipolar, etc.) by selectively closing the appropriate combination ofswitches (not shown) as is known in the art.

Atrial sensing circuits 82 and ventricular sensing circuits 84 may alsobe selectively coupled to the right atrial lead 20, coronary sinus lead24, and the right ventricular lead 30, through the switch 74 fordetecting the presence of cardiac activity in each of the four chambersof the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR.SENSE) sensing circuits, 82 and 84, may include dedicated senseamplifiers, multiplexed amplifiers, or shared amplifiers. The switch 74determines the “sensing polarity” of the cardiac signal by selectivelyclosing the appropriate switches, as is also known in the art. In thisway, the clinician may program the sensing polarity independent of thestimulation polarity.

Each sensing circuit, 82 and 84, preferably employs one or more lowpower, precision amplifiers with programmable gain and/or automatic gaincontrol, bandpass filtering, and a threshold detection circuit, as knownin the art, to selectively sense the cardiac signal of interest. Theautomatic gain control enables the device 10 to deal effectively withthe difficult problem of sensing the low amplitude signalcharacteristics of atrial or ventricular fibrillation. The outputs ofthe atrial and ventricular sensing circuits, 82 and 84, are connected tothe microcontroller 60 which, in turn, are able to trigger or inhibitthe atrial and ventricular pulse generators, 70 and 72, respectively, ina demand fashion in response to the absence or presence of cardiacactivity in the appropriate chambers of the heart.

For arrhythmia detection, the device 10 utilizes the atrial andventricular sensing circuits, 82 and 84, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used herein“sensing” is reserved for the noting of an electrical signal, and“detection” is the processing of these sensed signals and noting thepresence of an arrhythmia. The timing intervals between sensed events(e.g., P-waves, R-waves, and depolarization signals associated withfibrillation which are sometimes referred to as “F-waves” or“Fib-waves”) are then classified by the microcontroller 60 by comparingthem to a predefined rate zone limit (i.e., bradycardia, normal, lowrate VT, high rate VT, and fibrillation rate zones) and various othercharacteristics (e.g., sudden onset, stability, physiologic sensors, andmorphology, etc.) in order to determine the type of remedial therapythat is needed (e.g., bradycardia pacing, anti-tachycardia pacing,cardioversion shocks or defibrillation shocks, collectively referred toas “tiered therapy”).

Cardiac signals are also applied to the inputs of an analog-to-digital(A/D) data acquisition system 90. The data acquisition system 90 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device102. The data acquisition system 90 is coupled to the right atrial lead20, the coronary sinus lead 24, and the right ventricular lead 30through the switch 74 to sample cardiac signals across any pair ofdesired electrodes.

The microcontroller 60 is further coupled to a memory 94 by a suitabledata/address bus 96, wherein the programmable operating parameters usedby the microcontroller 60 are stored and modified, as required, in orderto customize the operation of the stimulation device 10 to suit theneeds of a particular patient. Such operating parameters define, forexample, pacing pulse amplitude, pulse duration, electrode polarity,rate, sensitivity, automatic features, arrhythmia detection criteria,and the amplitude, waveshape and vector of each shocking pulse to bedelivered to the patient's heart 12 within each respective tier oftherapy.

Advantageously, the operating parameters of the implantable device 10may be non-invasively programmed into the memory 94 through a telemetrycircuit 100 in telemetric communication with the external device 102,such as a programmer, transtelephonic transceiver, or a diagnosticsystem analyzer. The telemetry circuit 100 is activated by themicrocontroller by a control signal 106. The telemetry circuit 100advantageously allows intracardiac electrograms and status informationrelating to the operation of the device 10 (as contained in themicrocontroller 60 or memory 94) to be sent to the external device 102through an established communication link 104.

In the preferred embodiment, the stimulation device 10 further includesa physiologic sensor 108, commonly referred to as a “rate-responsive”sensor because it is typically used to adjust pacing stimulation rateaccording to the exercise state of the patient. However, thephysiological sensor 108 may further be used to detect changes incardiac output, changes in the physiological condition of the heart, ordiurnal changes in activity (e.g., detecting sleep and wake states).Accordingly, the microcontroller 60 responds by adjusting the variouspacing parameters (such as rate, AV Delay, V-V Delay, etc.) at which theatrial and ventricular pulse generators, 70 and 72, generate stimulationpulses.

The stimulation device additionally includes a battery 110 whichprovides operating power to all of the circuits shown in FIG. 2. For thestimulation device 10, which employs shocking therapy, the battery 110must be capable of operating at low current drains for long periods oftime, and then be capable of providing high-current pulses (forcapacitor charging) when the patient requires a shock pulse. The battery110 must also have a predictable discharge characteristic so thatelective replacement time can be detected. Accordingly, the device 10may employ lithium/silver vanadium oxide batteries, as are known in theart.

As further shown in FIG. 2, the device 10 is shown as having animpedance measuring circuit 112 which is enabled by the microcontroller60 via a control signal 114. The impedance measuring circuit 112 is notcritical to the present invention and is shown for only completeness.

In the case where the stimulation device 10 is intended to operate as animplantable cardioverter/defibrillator (ICD) device, it must detect theoccurrence of an arrhythmia, and automatically apply an appropriateelectrical shock therapy to the heart aimed at terminating the detectedarrhythmia. To this end, the microcontroller 60 further controls ashocking circuit 116 by way of a control signal 118. The shockingcircuit 116 generates shocking pulses of low (up to 0.5 joules),moderate (0.5-10 joules), or high energy (11 to 40 joules), ascontrolled by the microcontroller 60. Such shocking pulses are appliedto the patient's heart 12 through at least two shocking electrodes, andas shown in this embodiment, selected from the left atrial coilelectrode 28, the RV coil electrode 36, and/or the SVC coil electrode38. As noted above, the housing 40 may act as an active electrode incombination with the RV electrode 36, or as part of a split electricalvector using the SVC coil electrode 38 or the left atrial coil electrode28 (i.e., using the RV electrode as a common electrode).

Cardioversion shocks are generally considered to be of low to moderateenergy level (so as to minimize pain felt by the patient), and/orsynchronized with an R-wave and/or pertaining to the treatment oftachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range of 5-40joules), delivered asynchronously (since R-waves may be toodisorganized), and pertaining exclusively to the treatment offibrillation. Accordingly, the microcontroller 60 is capable ofcontrolling the synchronous or asynchronous delivery of the shockingpulses.

As may be further noticed with reference to FIG. 2, the device 10further includes a rate circuit 62 and a sensing control circuit 64. Thedevice 10 may be programmed to provide pacing pulses in any one of aplurality of pacing modes including single chamber, bichamber coupled orbichamber sequential pacing modes. Any one of the electrodes previouslydescribed may be employed for this purpose as may the case electrode 40.Preferably, in accordance with this embodiment, the device is programmedto provide biventricular pacing with the ventricular pulse generator 72being coupled to both electrode 32 and electrode 26. Using the case 40as a return electrode, pacing pulses may be applied to these electrodesand hence the right and left ventricles. The pacing pulses may beprovided simultaneously (biventricular coupled) or sequentially. In thisembodiment, the pacing pulses are provided in a trigger mode wherein anintrinsic activation of either chamber results in the immediateapplication of the simultaneous pacing pulses to both chambers. Further,the ventricular sense amplifier 84 is likewise coupled to theseelectrodes for sensing the intrinsic activations, and blanked duringdelivery of the pacing pulses.

The above configuration and operating mode maintained as long as thepacing rate remains below the maximum triggering rate (MGR). If the ratecircuit 62 determines a sensing rate exceeding the MGR, a ventriculartachycardia may be present. To avoid the possible double counting ofventricular events by the arrhythmia detector, in accordance with thisembodiment, the sensing control circuit 64 causes the switch 74 tocouple the ventricular sense amplifier 84 to only the right ventriclewith, for example, electrode 32 for sensing only in the right ventricleof the heart. This sensing configuration is maintained until the ratecircuit 62 determines that the sense rate has been below the MGR for atleast a given number (Y) of heartbeats. This ensures that thetachycardia has ended and that it is safe to transition back tobiventricular sensing.

In accordance with this embodiment, the given number (Y) may be aconsecutive number or Y heartbeats out of the last Z heartbeats.Further, in accordance with this embodiment, Y may be ten (10) and theMTR may be 130 bpm, for example. For counting the Y heartbeats, the ratecircuit 62 includes a counter 66.

In FIG. 3, a flow chart is shown describing an overview of the operationand novel features implemented in one embodiment of the device 10. Inthis flow chart, the various algorithmic steps are summarized inindividual “blocks”. Such blocks describe specific actions or decisionsthat must be made or carried out as the algorithm proceeds. Where amicrocontroller (or equivalent) is employed, the flow chart presentedherein provides the basis for a “control program” that may be used bysuch a microcontroller (or equivalent) to effectuate the desired controlof the stimulation device. Those skilled in the art may readily writesuch a control program based on the flow charts and other descriptionspresented herein.

The process of FIG. 3 is initiated and completes on a beat-by-beatbasis. It initiates with activity block 120 wherein the rate circuit 62determines the current sense (trigger) rate. The process then advancesto decision block 122 wherein the rate circuit determines if the currentsense (trigger) rate is above the MGR. If it is, evidencing a potentialtachyarrhythmia, the process advances to activity block 124 wherein thecontrol circuit 64 causes the switch 74 to set the sensing configurationfor sensing only in the right ventricle. The process then advances toactivity block 126 to reset counter 66 before returning.

If in decision block 122 it is determined by the rate circuit 62 thatthe current sense (trigger) rate is below the MGR, the process advancesto decision block 128. In decision block 128, the rate circuitdetermines if the count in counter 66 is equal to or greater than thegiven number (Y). If it is not, indicating that the device has beensensing only in the right ventricle and that the trigger rate has notbeen below the MGR for enough heartbeats to enable a sensingconfiguration transition, the process advances to activity block 130 toincrement counter 66. The process then returns.

If in decision block 128 it is determined that the counter is equal toor greater than the given number (Y), the process advances to activityblock 132. Here, the control circuit 62 causes the switch 74 to couplethe sense amplifier 84 for sensing activity of both the right and leftventricles if previously, sensing of only right ventricular activity wasbeing performed, or to maintain a prior biventricular sensingconfiguration. Hence, the transition from single chamber sensing tobiventricular sensing has hysteresis to assure that the possibletachyarrhythmia has lapsed before the transition occurs.

While this embodiment has been primarily directed to biventricularpacing and sensing it will be appreciated by those skilled in the artthat the foregoing could be applied equally as well to the correspondingchambers of the right and left atria. Biatrial sensing may be performedwith electrodes 22 and 27. Single chamber sensing only may be performedwith electrode 22.

While the invention has been described by means of specific embodimentsand applications thereof, it is understood that numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the spirit and scope of the invention. It is therefore tobe understood that within the scope of the claims, the invention may bepracticed otherwise than as specifically described herein.

1. In an implantable cardiac stimulation device, a method of sensing activity of a heart comprising: determining a cardiac rate associated with interaction of the device and the heart; and sensing cardiac activity of the heart, the sensing step including a first sensing step of sensing cardiac activity of corresponding chambers of the heart when the cardiac rate is below a given rate and a second sensing step of sensing cardiac activity of a single chamber of the heart when the cardiac rate is above the given rate.
 2. The method of claim 1 including the further step of providing pacing pulses to the corresponding chambers of the heart responsive to sensing cardiac activity of the heart.
 3. The method of claim 2 wherein the providing step includes providing simultaneous pacing pulses to the corresponding chambers of the heart.
 4. The method of claim 2 wherein the providing step includes providing sequential pacing pulses to the corresponding chambers of the heart.
 5. The method of claim 2 wherein the providing step includes providing the pacing pulses in a trigger mode.
 6. The method of claim 5 wherein the given rate is below a maximum trigger rate.
 7. The method of claim 1 wherein the corresponding chambers are a right ventricle and a left ventricle of the heart.
 8. The method of claim 7 wherein the single chamber is one of the ventricles of the heart.
 9. The method of claim 8 wherein the single chamber is the right ventricle.
 10. The method of claim 1 wherein the corresponding chambers are a right atrium and a left atrium of the heart.
 11. The method of claim 10 wherein the single chamber is one of the atria of the heart.
 12. The method of claim 11 wherein the single chamber is the right atrium.
 13. The method of claim 1 wherein the determining step includes determining the cardiac rate for each heartbeat and wherein the sensing step further includes transitioning from sensing cardiac activity of the single chamber to sensing the cardiac activity of the corresponding chambers of the heart after the rate has been below the given rate for a predetermined number of heartbeats. 